The present invention relates to a method for operating a waste heat utilization device for an internal combustion engine of a motor vehicle. The invention also relates to an associated waste heat utilization device.
A waste heat utilization device for an internal combustion engine, in particular a motor vehicle, with a waste heat utilization circuit in which a working medium is circulated, is known from DE 10 2008 057 202 A1. A conveyor for conveying the liquid working medium against a high pressure is disposed in the waste heat utilization circuit. Furthermore, an evaporator for vaporizing the liquid working medium is disposed in the waste heat utilization circuit downstream of the conveyor. For this purpose the evaporator extracts heat from the internal combustion engine. In the waste heat utilization circuit an expansion machine for expanding the gaseous working medium to a low pressure is disposed downstream of the evaporator. A condenser for condensing the gaseous working medium is disposed downstream of the expansion machine in the waste heat utilization circuit. In the condenser heat is removed from the waste heat utilization circuit. This may be achieved for example by means of a cooling device which comprises a cooling circuit in which a coolant is circulated and which is coupled to the condenser for heat transfer. To this end a control unit may be provided for adjusting the quantity of working fluid circulating in the Clausius-Rankine circuit as a function of the current operational state of the Clausius-Rankine circuit, the control device being configured so that it adjusts the quantity of working fluid circulating in the Clausius-Rankine circuit by changing the condensation output of the condenser, in particular by changing the quantity of cooling fluid flowing through the heat exchanger. Furthermore, a collector for storing the liquid working medium is disposed in the waste heat utilization circuit upstream of the conveyor. In the known waste heat utilization device provision is made for integrating the collector in the condenser. Moreover provision is made for adjusting the waste heat utilization circuit according to the current operating point of the waste heat utilization circuit by changing the condenser output by means of a control valve in the cooling circuit. This adjustment effects a change in the working medium mass distribution in the waste heat utilization circuit.
Such an adjustment of the waste heat utilization circuit only by means of the adjustment of the condenser output on the basis of the current operating point of the waste heat utilization circuit is slow and only optimizable to a limited extent, for example with regard to a high output yield.
The working medium mass distribution is definitive for optimized high pressure in the waste heat utilization circuit and thus for the gradient between high and low pressure at the expansion machine, the optimized output occurring at the expansion machine. However, in vehicle applications the problem exists that the internal combustion engine which is used as a heat source for operating the waste heat utilization device, namely for delivering heat to the evaporator, is frequently operated transiently or has many different steady-state operating points, at which in particular different amounts of heat are provided for output to the waste heat utilization device.
However, if in such transient states of the internal combustion engine the heat available to the evaporator changes, the mass of the gaseous working medium in the waste heat utilization circuit changes. Consequently a change occurs to the high pressure in the gaseous working medium, that is to say between the evaporator and the expansion machine. This leads to an “imbalance” in the system, which can only be compensated comparatively slowly by a conventional adjustment system, which reduces the efficiency of the waste heat utilization device and the output thereof.
With this as the starting point, the object of the present invention is to provide an improved embodiment of a waste heat utilization device or an associated method for operation thereof, which is characterized in particular by an improved overall efficiency or by a fast adaptation to fluctuations in the amount of heat available to the evaporator.
The method according to the invention is characterized in that a basic adjustment of the waste heat utilization circuit is provided, which as a function of a heat input into the working medium adjusts the mass flow rate at the conveyor and/or the ratio between high pressure and low pressure at the expansion machine, and that in addition to this a pilot control is provided, which recognizes a change of the operating point of the internal combustion engine and, when the operating point of the internal combustion engine changes, controls the condensation output of the condenser in accordance with a mass distribution of the working medium in the waste heat utilization circuit optimized to the new operating point.
The waste heat utilization device according to the invention is characterized by a basic adjustment device with a control unit which is connected to the conveyor and/or the expansion machine for controlling or adjusting the waste heat utilization device, and by a pilot control unit for pilot control of the condensation output of the condenser, with a control unit which acts on a cooling performance adjustment device and, when the operating point of the internal combustion engine changes, controls the condensation output of the condenser in accordance with a mass distribution of the working medium in the waste heat utilization circuit optimized to the new operating point.
The invention is based on the general idea of combining a slow basic adjustment with a fast-acting load change adjustment or load change pilot control. A pump and/or expander adjustment is provided as the basic adjustment. The conveyor is typically a volumetric pump which is characterized in that its flow rate is independent of the counter-pressure and in particular is only dependent upon its speed. The high pressure in the waste heat utilization circuit is determined in this case by the expansion machine.
In addition, during a transient operational state of the internal combustion engine, that is to say in the event of a change from an old operating point to a new operating point, the condensation output of the condenser can be temporarily changed so that with regard to the new operating point a displacement of the mass distribution occurs in the waste heat utilization circuit. Thus already in the context of a pilot control, fluctuations and imbalances, which are expected due to the change of operating point, can be at least roughly compensated. Such a pilot control acts and must not react like a conventional adjustment, since the time lag until an adjusting intervention can take place is comparatively long because of the inertia of the thermal mass of the waste heat utilization circuit. In particular according to the method presented here the condensation output of the condenser can be temporarily changed so that a distribution of the mass of the liquid working medium and the mass of the gaseous working medium in the waste heat utilization circuit shifts from an old mass distribution value associated with the old operating point to a new mass distribution value associated with the new operating point.
The operating points of the internal combustion engine may be defined for example by the speed of the internal combustion engine and/or the load applied to the internal combustion engine. In particular such parameters which correlate with the operating point, such as for example speed and load, are available in an engine control unit provided for operation of the internal combustion engine.
According to a preferred embodiment the association of the mass distribution values with the operating points of the internal combustion engine with a view to optimized output can take place at the expansion machine. This may for example take place by constant attempts to set an optimal high pressure or an optimal gradient between high and low pressure. Additionally or alternatively it may also be provided that the association between the operating point of the internal combustion engine and the mass distribution value is implemented by means of at least one characteristic curve or at least one characteristic field. Characteristic fields or characteristic curves have proved worthwhile in practice and can in particular be determined empirically in order to obtain reliable associations between the mass distribution values and the operating points.
For a change of the condensation output of the condenser, according to an advantageous embodiment provision may be made for altering the cooling performance of a cooling unit which is coupled to the condenser for heat transfer. In an embodiment of the method the control of the condensation output of the condenser takes place by means of a change of a mass flow of a coolant in the condenser. A change of the supply of heat to the waste heat utilization circuit, which is expected on the basis of a change of operating point of the internal combustion engine, prompts the pilot control to change the heat removal from the waste heat utilization circuit. That is to say the pilot control adapts the external effects of the waste heat utilization circuit to one another at an early stage. Therefore the basic control of the waste heat utilization circuit can operate without fast and extensive change.
According to another advantageous embodiment, in order to change the cooling performance of the cooling unit, the mass flow rate of a coolant which circulates in a cooling circuit coupled to the condenser for heat transfer can be changed. Furthermore, in order to change the mass flow rate of the cooling agent, provision may optionally be made for changing the conveying capacity, in particular the speed, of a conveying device which is disposed in the cooling circuit and drives the coolant. In this embodiment of the device the cooling performance adjustment device is designed as an adjustable cooling circuit pump. This is typically already present in the cooling circuit, so that no additional component is required.
In an alternative or supplementary embodiment of the device the cooling performance adjustment device is designed as an adjustable bypass of the condenser cooling circuit. Thus the amount of cooling medium which cools the condenser per unit of time can be adjusted.
In one embodiment of the method the change in the mass distribution of the working medium is detected by means of a filling level measuring device disposed in the collector. For this purpose in one embodiment of the device a filling level measuring device is provided in the collector. This constitutes a simple possibility for establishing the success of the method and reaching a target value. Alternatively a theoretical determination of the filling level is possible by means of a characteristic field or by calculation.
In an embodiment of the device a temperature measuring device is provided for measuring at least a temperature of a coolant which circulates in a cooling circuit of a cooling unit provided for removing heat from the condenser and the control unit is coupled to the temperature measuring device and is configured and/or programmed for changing the cooling performance of the cooling unit. Thus the temperature of the coolant can be detected in order to determine the mass of coolant required for a desired cooling performance per unit of time.
Advantageously, therefore, the cooling performance of the coolant can be adjusted so that the mean value of the feed line temperature and return line temperature of the coolant at the condenser is below the dew point of the working medium in particular by a predetermined value. In an embodiment of the device, therefore, the temperature measuring device has a feed line temperature sensor for detecting the temperature of the coolant before the condenser and a return line temperature sensor for detecting the temperature of the coolant after the condenser. The difference between the feed line temperature and the return line temperature corresponds to the heat input of the waste heat utilization circuit into the cooling medium.
The waste heat utilization device according to the invention is equipped with a control unit which is coupled to a control device, in particular an engine control unit, of the internal combustion engine and which serves for controlling and/or adjusting a condensation output of the condenser. In this case the control unit is programmed or configured so that in the event of a change from an old operating point of the internal combustion engine to a new operating point of the internal combustion engine the condensation output of the condenser is temporarily changed so that a distribution of the mass of the liquid working medium and the mass of the gaseous working medium in the waste heat utilization circuit shifts from an old mass distribution value associated with the old operating point to a new mass distribution value associated with the new operating point.
In an advantageous modification a cooling unit can be provided for removing heat from the condenser, wherein the control unit is then configured or programmed for controlling and/or adjusting a cooling performance of the cooling unit as a function of the filling level of the liquid working medium in the collector.
A particularly compact construction for the waste heat utilization device can be achieved if a collector, which is provided in the waste heat utilization circuit upstream of the conveyor for storage of the liquid working medium, is formed in a housing of the condenser. The collector and condenser are then integrated into one another.
Further important features and advantages of the invention are disclosed in the drawings and the associated description of the figures of the drawings.
It will be understood that the features referred to above and still to be explained below can be used not only in the respectively specified combinations but also in other combinations or alone without departing from the scope of the present invention.
Preferred embodiments of the invention are illustrated in the drawings and are explained in greater detail in the following description.